This invention relates to the field of tone detectors and, more particularly, to the use of low frequency tone detectors with stereophonic signals.
Many tone detectors are known which consist mainly of low-pass or bandpass filters, active or passive, with the output rectified to obtain a DC mode-controlling signal. One such system is shown in U.S. Pat. No. 4,159,398, assigned to the assignee of the present invention. The patent discloses AM stereophonic transmission and reception of a signal including a very low frequency stereo presence tone. One problem which can arise with a simple filter/rectifier combination is that false stereo indication signals can be triggered by noise or other signals at or very near the tone frequency. The user of a stereophonic receiver having a falsing problem may see a flickering indicator lamp and hear the audio move back and forth between the "middle" or monophonic position and the separate stereo positions. Any tone detector added to a receiver which includes "scanning" should provide for an early "non-detect" in order to avoid slowing down the scan.